This invention relates to the process for the production of high concentration caustic alkali through ion exchange membrane electrolysis from alkali halide solution at high output efficiency. As is well known, halogen, especially chlorine, and caustic alkali are basic and important materials which are used broadly in various industrial manufacturing firms.
It is also well known to use selectively operating ion-permeable membranes for the separation between the anolyte and the catholyte relating to the aqueous salt solution under electrolysis. When, however, it is desired to execute the electrolysis for the production of caustic alkali by use of a single ion-permeable membrane with 80% or still higher current efficiency, the resulting caustic alkali concentration will be 20% or so at the maximum. It is, therefore, necessary to adopt a condensing step to increase the concentration of the thus obtained caustic alkali, because consumers generally desire a highly concentrated caustic alkali for the reason of easier transportation and the like. It is nowadays, therefore, a goal of the electrolysis to obtain more than 30%, preferably 40% or so of a caustic alkali through the one time electrolysis without using a condensation step.
It is, therefore, the main object of the present invention to provide an improved electrolytic process for the manufacture of high concentration caustic alkali from aqueous lower concentration alkali halide solution using a simpler bath structure.
A further object is to provide the process of the above kind for the production of industrial caustic soda from sea water or the like low concentration brine.
For carrying out the process according to this invention, the electrolytic vessel means may preferably comprise an anodic chamber, an intermediate chamber and a cathodic chamber arranged in series one after another and separated by anodic (i.e., cationic selective) ion exchange membranes. When necessary, the intermediate chamber may be fed with water or aqueous low concentration caustic alkali solution, while the cathodic chamber is not fed with water at all, so as to provide a capability of producing high concentration caustic alkali as the product.
It has already been disclosed in U.S. Pat. No. 3,933,603 that in, using a trichamber-type electrolytic vessel of a similar kind as above, both intermediate and cathodic chambers are fed with water or aqueous low concentration caustic alkali solution, and the caustic alkali concentration in the intermediate chamber is set to a low value, so as to obtain a high current efficiency.
In the case of the present invention for the manufacture of high concentration caustic alkali, the purpose thereof can be surprisingly fulfilled by feeding no water or no aqueous component to the cathodic chamber of the trichamber-type electrolytic vessel, except during the starting stage of the electrolytic operation, resulting in improved current efficiency.
In order to accomplish a twin chamber style ion exchange membrane electrolytic process and in order to improve the electrolytic efficiency, therein it is necessary to exclusively convey selectively the alkali metal ions such as Na.sup.+, K.sup.+ and the like to the cathodic chamber, so as to convert, in the cathodic chamber, these ions into caustic alkali. When the concentration of the caustic alkali in the cathodic chamber becomes high, part of OH.sup.- -ions can move towards the anodic chamber by the unavoidable diffusion. The thus transferred OH.sup.- -ions give rise to oxidation at the anode or to undesirable reaction with other ions prevailing therein which invites an inferior current efficiency. Complete prevention of such transfer of OH.sup.- -ions could not be realized so far as nowadays available ion exchange membranes are used.
In the case of the filter diaphragm electrolysis, a large quantity of brine will flow from the anodic chamber towards the cathodic chamber, thereby checking the diffusion of OH.sup.- -ions towards the anodic chamber which would otherwise lower the current efficiency.
In the case of tri-chamber mode electrolytic process, the alkali metal ions passed through the ion exchange membrane arranged between and to separate the anodic and intermediate chambers will become caustic alkali in the intermediate or cathodic chamber, while only the OH.sup.- -ions which have passed through the same membrane will act in the anodic chamber to separate oxygen and the like gases, thereby inviting a substantial reduction of the current efficiency. Therefore, the current efficiency in this electrolytic bath in the entirety will be decided by the operating condition of the ion exchange membrane separating the anodic chamber from the intermediate chamber. Therefore, it is thus highly advantageous to keep the alkali concentration at low value, so as to improve the current efficiency of the said ion exchange membrane. Based upon such observation, it has already been proposed and brought into practice to feed water or aqueous caustic alkali solution to the intermediate and cathodic chambers, so as to produce in the former, low concentration caustic alkali, and to generate higher concentration caustic alkali in the cathodic chamber.
In the apparatus for carrying out the process according to this invention, there are again provided both intermediate and cathodic chambers, and when necessary, water or aqueous caustic alkali solution is fed to the intermediate chamber. The cathodic chamber is arranged so that the caustic alkali and/or water flow thereinto exclusively by the osmosis through the ion exchange membrane from the intermediate chamber. In this way, highly concentrated caustic alkali is formed in the cathodic chamber.
In the case of the electrolytic treatment of common salt solution with use of the tri-chamber mode ion exchange membrane electrolytic bath, the concentration of caustic soda solution prevailing in the intermediate chamber is determined, according to the following well known method, by the sum of the amount of caustic soda which is formed by the OH.sup.- -ions flowed from the cathodic chamber, less those further flowed away towards the anodic chamber, the thus resulted residual OH.sup.- ions reacting with Na.sup.+ -ions prevailing in the intermediate chamber, (if the latter chamber be fed with caustic soda solution, the sum must be supplemented with the amount of caustic soda contained in the solution), and the quantity of water fed to the intermediate chamber. According to our information, however, it has been found that an appreciable amount of water passes through the ion exchange membrane and affects the alkali concentration prevailing in the intermediate chamber.
Based upon the above observation, we propose that one should not feed water or any aqueous solution including caustic alkali solution to the cathodic chamber since the formation of caustic alkali at the latter chamber depends exclusively upon such alkali metal ion and water which have transferred from the intermediate chamber and through the ion exchange membrane, so as to provide higher concentrated caustic alkali as possible. As a result of further experimentation, the above proposition has been shown to be true and a much higher concentration caustic alkali solution than expected has been obtained with an extraordinarily high current efficiency.